Go over to the Android Studio page, https://developer.android.com/sdk/installing/studio.html, and click the DOWNLOAD ANDROID STUDIO button:
On the popup that appears, read and accept the terms and conditions and click the DOWNLOAD ANDROID STUDIO FOR MAC button:
The name of the button is different depending on the operating system you're using.
The download will begin and you'll be redirected to an instructions page (https://developer.android.com/studio/install).
Follow the instructions specified for your operating system to install Android Studio. When the installation is complete, open Android Studio and start the setup process.